FindBoost: Add support for Boost 1.71

Run Utilities/Scripts/BoostScanDeps.cmake with the Boost 1.71.0 sources to compute dependencies. They are the same as Boost 1.70.

Also unwrap compatibility INTERFACE targets for legacy variables. Since !3438 (merged) we extract information from imported targets provided by upstream BoostConfig.cmake files. However, upstream Boost 1.71 provides some imported targets only for compatibility. They are just INTERFACE libraries that wrap around the real target named by INTERFACE_LINK_LIBRARIES. Unwrap this layer so we can extract the real imported location.

Fixes: #19656 (closed), #19658 (closed)